This invention relates to electro-optical display devices, such as active matrix liquid crystal display (AMLCD) devices, and more particularly relates to such devices incorporating storage capacitors.
Reflective high density active matrix liquid crystal displays (AMLCDs) on silicon are being considered for high resolution color projection systems, due to the fact that the matrix structure of row and column electrodes, switches and storage capacitors can all be integrated on the silicon substrate below the reflective pixel electrodes, thus enabling the high pixel density required for such high resolution displays.
For example, the 1280 columns and 1024 rows required for HDTV requires that more than 1.3 million pixels must fit onto an area of about 2 square cm. In order to have a sufficient optical aperture for acceptable brightness and contrast ratio, the pixels must be as large as possible. With the reflective AMLCD structure, inter-pixel spacings of around 1 micron are possible, which provides the pixel area necessary to achieve an optical aperture sufficient for acceptable performance.
A typical defect density in the sub-micron CMOS processing techniques used to fabricate such AMLCDs is around one pixel defect per square centimeter. However, in such high resolution displays, one pixel defect in the entire array is sufficient to result in rejection of the display.
The most common defects are cross-over shorts, pixel switch failure and storage capacitor shorts. For example, capacitor shorts reduce the ability of the pixel to hold its display information during the interval between addressing and readdressing, affecting, for example, grey scale rendition.
In order to overcome such defects, and thereby achieve high display quality with high manufacturing yield, redundant structures have been designed into such AMLCDs, including redundant switches and electrodes.
In such displays, a large area (almost 75% of the pixel area) and a thin dielectric layer (400 nm) are needed for the pixel storage capacitor to provide the required capacitance (10 times the capacitance of the liquid crystal material) to hold the pixel voltage over the frame period. Due to this large area and thin dielectric layer, chances of a short are somewhat higher than for the pixel switches and other circuit elements.
A redundant storage capacitor structure is described in published international patent application WO 91/02372. In that case, the pixel capacitor can be a junction or oxide type capacitor or a combination thereof. Since the combination structure is a multilevel multilayer structure, the fabrication thereof requires multiple processing steps, which increases manufacturing cost and decreases yield.